PPPL-led team completes work on JET alpha detector
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Studying the behavior of alpha particles produced in fusion plasmas is of paramount importance for ITER and other advanced fusion devices in which these particles are expected to be the predominant source of plasma heating. An international team led by PPPL physicist Doug Darrow recently completed work at PPPL on the construction of diagnostic equipment that will be used to measure alpha particles and other energetic particles ejected from the plasma in the Joint European Torus (JET) in Culham, England. The new diagnostic arrived at JET on April 13.
Neutrons and alpha particles are produced when deuterium (D) and tritium (T) ions fuse. The neutrons carry away about 80% of the fusion energy produced, the alphas the rest. The positively charged alpha particles (helium-4 nuclei) can impart energy to the plasma, heating it. "PPPL did a good bit of work on alpha detection for the deuterium-tritium experiments on TFTR. A lot of good results came out of that work, and that's what sold JET on getting into this in earnest," noted Darrow, who was involved in the TFTR measurements.
The diagnostic equipment, built at PPPL, is one of two different kinds of JET alpha particle detectors being supplied under a collaboration among researchers from the UKAEA, the Max Planck Institute for Plasma Physics (IPP) in Greifswald, Germany, and the Colorado School of Mines in the U.S. PPPL's effort began in early 2002. Since then, the Laboratory has received approximately $1.5 million from the DOE for the design and fabrication of one of the two detectors. In addition the IPP has invested roughly $1.0 million for the design and fabrication of the second detector at Greifswald.
Lost Alphas
The collaborations primary interest is the measurement of alpha particles that exit the plasma before having a chance to heat it. "Alphas and other energetic particles can drive instabilities in the plasma which cause the alphas to be ejected. Studies on JET using these lost alpha diagnostics will provide new insights into the causes and nature of instabilities driven by alpha particles and other energetic particles," noted Darrow. The lost alphas can also have a deleterious effect on the first wall of a fusion reactor. They tend to strike in specific locations, which can result in hot spots that affect the integrity of the first wall. So it is critically important to understand the loss mechanism of the alphas.
The two diagnostics being built under the collaboration will use different detection methodologies that together provide complementary information about the lost alphas. According to Doug, "The detector that we just finished building here at PPPL uses Faraday cups -- metal foils that collect alpha particles coming out of the plasma. We measure the electrical current coming from each foil to infer the number of alphas striking the foil. The current will be very tiny, but it is measurable." Prof. Ed Cecil of the Colorado School of Mines pioneered the thin foil Faraday cup approach. PPPL's Bob Ellis, Mike Messineo, Dave Miller, and Joe Frangipani have, respectively, done the engineering, drafting, and technical work required to design and build the Faraday cup diagnostic.
The other diagnostic, being built at the Max Planck Institute for Plasma Physics, is a scintillator detector that uses metal plates covered with a phosphor of the kind found on a TV monitor, or any cathode ray tube. The alpha particles hit the phosphor creating light. A CCD camera records successive images from the phosphor. The images will indicate where and how many alpha particles are striking the plate. The Faraday cups will be deployed at multiple locations around the plasma, comprising an array of detectors that will measure where the alphas are coming from. The scintillator detector will provide a more precise measurement of the energy of the alphas and their directions, or pitch angles, but at only one location.
The directional information from the scintillator detector pinpoints the actual orbits of the alphas as they leave the plasma, and physicists are hoping to use this data to determine what's going on inside the plasma. At present, researchers do not have a reliable method for measuring directly the alpha population inside the plasma. Consequently, they would have to calculate the alpha particle heating profile based on other experimental data. Until direct measurements are available, physicists plan to extract as much information as possible from the location and orbits of the lost alphas as they leave the plasma.
Experiments to Begin this Fall
Experimental operations on JET employing the new alpha particle diagnostics are scheduled to begin in November of this year. While another round of JET D-T experiments is not anticipated in the near term, initial experiments with the new diagnostics will be performed in plasmas heated by a variety of energetic ions. In deuterium plasmas, JET has a variety of ways of generating particles with energies approaching the 3.5-million-electron-volt (MeV) alpha particles produced in D-T fusion reactions. The new alpha particle detectors can measure these particles as well. For example, D-D fusion reactions yield 3-MeV protons and 1-MeV tritons. Additionally, JET has strong radiofrequency (ion cyclotron) heating that can energize hydrogen ions (protons) in the plasma, raising them into the MeV range. JET also has the ability to heat 150-kilo-electron-volt (KeV) helium neutral-beam ions with RF power, kicking them up to the MeV range to simulate the behavior of D-T fusion produced alphas. Consequently PPPL and JET fusion scientists expect the alpha particle detectors to provide new insights into the dynamics of energetic particles and how they interact with the plasma over a wide range of conditions.
Relevance to ITER
ITER's mission is to produce "burning plasmas" in which heating is predominantly supplied by alpha particles produced by D-T reactions, as opposed to auxiliary heating from neutral-beam injection or radiofrequency waves in present experiments. The study of burning plasmas is essential for the development of practical fusion power. Experiments planned on JET in the next few years will add much needed knowledge to support the work planned on ITER. But ITER's success depends on more than just the physics results coming from JET collaborations. It also hangs on the ability of scientists and engineers from around the world to work together effectively to design and build the needed equipment. "JET is an experiment in two ways. First it is a scientific experiment approaching a reactor-relevant scale that is not available in the U.S., but it is also an experiment in multi-national collaboration. A consortium of fusion partners has developed, not without pains, methods by which this can be done effectively and efficiently. By engaging in the JET collaboration, we prepare ourselves for ITER and beyond. The success of our collaboration bodes well for the future of fusion research," noted Raffi Nazikian, who oversees international collaborations within PPPL's office of Off-site Research.